Interestingly, your reasoning echoes in Buffy and Willow's dialogue in issue 11:

Willow: Not killing humans is what separates us from the bad guys.
Buffy: No, not being bad is what separates us from the bad guys.

It's unclear how we should react on Buffy's words. After all, she *did* kill humans - the knights of Byzantium were human, Spike's chip worked on them. But, ultimately, Buffy was right to kill them, because it was self-defence.